1. Technical Field of the Invention
The present invention relates generally to the manufacture of structural components from a heat-treatable alloy, particularly an aluminium alloy, by plate machining, as well as to structural components per se. These structural components may be used, for example, in aircraft construction.
2. Description of Related Art
To obtain aircraft structural components having superior mechanical strength as required for many such applications, at least two different approaches are currently used in the industry.
According to a first approach, rolled products typically between 10 mm and 40 mm thick (hereinafter “medium plate”), are employed which rolled products are typically in a temper corresponding to the structural component's desired end use. Such medium plates are stiffened by anchoring, for example, riveting, and/or by stiffeners constituted by extruded profiles, for example, profiles of T-shaped cross section.
According to a second approach, stiffeners are machined directly in a plate of more substantial thickness, typically between 30 mm and 200 mm or more (hereinafter “thick plate”), which is also in a temper corresponding to a structural component's desired end use. This process is what is known as “plate machining.”
Making a structural component by assembling medium plates and profiles requires a very large number of riveting operations which, when carried out under the rigorous conditions of reliability required for an aircraft structural component, involve substantial cost. Making an integrated structural component by plate machining certainly uses more metal, since a sizeable fraction of the plate is reduced to machining scrap such as turnings and slivers, but in return, plate machining allows expensive riveting operations to be cut to a minimum.
The availability of high speed machining technology, of the order of 5,000 to 15,000 revolutions per minute, appreciably redefines the economics of design mode selection, since the duration of the machining operation is substantially reduced. This makes it possible to be able to machine increasingly complex shapes in economically affordable conditions. This is true both for parts of about 1 metre in size, and for parts of very large size, possibly over 20 m long and over 3 m wide.
The second approach (plate machining) suffers, however, from some drawbacks. As mentioned above, the plate is generally prepared in the temper corresponding to its end use prior to machining. This is because, according to the prior art, no thermo-mechanical treatment can be applied after machining. More particularly, this final temper has been obtained by solution heat treatment and quenching. In fact, two physical mechanisms restrict the quenching rate in a plate: (i) the thermal conductivity of the material constituting the plate, and (ii) the heat exchange between the surface of the plate and the quenching medium. The result is that the mechanical properties of the quenched plate vary according to its thickness.
For this reason, a number of mechanical characteristics become less acceptable as you move away from the surface of the plate. That is, the interior sections of the plate do not have the same mechanical properties as exterior portions, and in fact, the interior portions will generally have inferior mechanical properties as compared to sections that are closer to the exterior. Prior processes involved machining that effectively removed the areas in which the quenched plate possesses better mechanical characteristics. Further, any action on structural components made from such plates under normal operating conditions will necessarily involve areas of metal with mechanical properties which may vary considerably as a function of depth relative to the area near the original surface of the plate. The properties of structural components (or parts) made from such plates are, as a precaution, calculated on the basis of quite conservative actual performance models (i.e., based on the interior properties) of the part. This prevents one from taking advantage of the actual properties of the material when parts are sized. Another drawback of the plate machining process approach according to the prior art lies in the fact that quenched plates can, even after controlled stretching, house residual stresses that cause the parts to distort during machining.
According to a third approach, structural components are made with integrated stiffeners directly by extrusion. This approach suffers from several drawbacks, and is not often utilized. That is, in order to obtain extruded profiles of sufficiently significant width, very powerful extrusion presses must be used which have a very high operating cost. The maximum width that is attainable remains well below the width of a traditional rolled sheet or plate. Furthermore, some alloys do not lend themselves well to extrusion. And finally, the microstructure of an extruded part, and more particularly of an extruded profile, is not homogeneous, neither over the cross-section of the profile nor over the length of the profile; this, in turn, leads to disadvantages in terms of quality assurance.
Various means have also been proposed to control the distortions of the product or its mechanical properties. Several patents seek to optimise the quenching process in order to minimise the distortions of the metallurgical products when they are quenched. These processes generally seek to compensate for distortion by inhomogeneous cooling during quenching.
The German patent DE 955 042 (Friedrichshüitte Aktiengesellschaft) describes a horizontal quenching process wherein the edges of the sheet are cooled more highly than the centre and the bottom side more highly than the top side.
The patent EP 578 607 seeks to optimise the process of quenching extruded profiles by individual or grouped control of the water spray nozzles. By using a computer controlled device, it is possible in principle to adapt the positions of the nozzles to each profile, with final adjustment remaining empirical. The patent EP 695 590 develops a similar idea in respect of plate quenching.
The patent application WO 98/42885 (Aluminium Company of America) describes a combined water quenching and air quenching process for reducing the distortion of thin plates at quenching, and improving their static mechanical characteristics.
The French patent 1,503,835 (Commissariat àl'Energie Atomique/French Atomic Energy Authority) proposes increasing the quenching rate when the part is immersed in a cold liquid by applying a thin layer of low thermal conductivity that restricts the evaporation of the quenching environment.
The French patent 2 524 001 (Pechiney Rhenalu) proposes applying a coating to some surfaces of the product which conducts heat less well than the subjacent metal. By thus improving control of the cooling rate, it would be possible to avoid impairing product performance. This rather cumbersome process has a number of drawbacks. It is restricted to sheets or profiles of substantially constant thickness; in the case of aluminium alloys, this thickness should not exceed about 15 mm. The coatings proposed in this patent may well pollute the quenching liquid tank.
Other approaches seek to reduce the sensitivity of aluminium alloys to quenching.